Protection devices of this type are found, in particular, in telecommunication networks where conductive lines connect subscribers to collective equipment of telephone exchange type. In such a network, the line connecting the subscriber to the exchange center may be mainly submitted to two types of disturbances resulting in overvoltages. A first type of disturbances, called short disturbances, relates to atmospheric phenomena such as lightning which is likely to hit overhead telephone lines. Overvoltages of atmospheric type are relatively short (a few hundreds of microseconds) with a strong current (a few tens of amperes). A second type of disturbances, called lasting disturbances, relates to risks for the line of conveying a current coming from the power supply system (the mains at 50 or 60 Hz). These disturbances may come from a direct contact with the mains, or from induction phenomena if the low-voltage telephone line is placed close to a high-voltage mains conductor. Overvoltage of mains type are relatively long (likely to reach several minutes) and are also likely to reach RMS currents of a few tens of amperes.
The equipment connected to telephone lines (subscriber station or telephone exchange) are usually provided with protection devices aiming at avoiding their destruction upon occurrence of such overvoltages.
FIG. 1 very schematically shows a conventional example of a device for protecting a user equipment (UE) 1, for example a telephone subscriber station. This device is divided into a protection device (PTC) 2 of series type and a protection device 3 of parallel type. The series protection device is connected between a first conductor T of the telephone line and a terminal 4 of access to the user equipment, as close as possible to this equipment. Parallel protection device 3 is connected between input terminals 4 and 5 of user equipment 1, terminal 5 being, for a user equipment, directly connected to a second conductor R of the telephone line.
Parallel protection device 3 mainly has the function of ensuring the protection against disturbances of atmospheric type, that is, of short duration. Its function is to pull the line current in the presence of an overvoltage. Parallel protection device 3 is most often formed of a brick-over component of bi-directional Schockley diode type. Such a component is known under trade name Trisil and has a current-vs.-voltage characteristic of the type illustrated in FIG. 2. This device appears as an open circuit as long as voltage V thereacross does not reach a threshold value Vb0, whatever its biasing. When this threshold voltage is reached, device 3 becomes a short-circuit enabling passing of a current I, thus clipping the voltage across the load. The opening back of device 3 is performed when the current flowing therethrough becomes lower than a hold current Ih.
Such a device is well adapted to the protection against overvoltages of short duration but is not capable of dissipating power for a duration on the order of several minutes. It is therefore associated with series protection device 2. Device 2 operates as a positive temperature coefficient resistor, that is, the higher the current flowing therethrough, the more the device heats up and the more its resistance increases. Device 2 may be sized so that from a given current threshold, its resistance is such that it isolates the line load. For example, device 2 is a polymer bar charged with conductive particles. When the polymer expands, the conductive particles contained therein separate from one another, which opens the circuit formed by this protective bar.
In the case where the equipment to be protected is a user equipment with no independent electric power supply, it is not grounded and parallel protection device 3 is then connected between the two line input terminals 4 and 5, as illustrated in FIG. 1.
In the case where the equipment to be protected is provided with a ground connection, as is the case for example for a telephone exchange, the protection device described in relation with FIG. 1 is duplicated for each conductor of the telephone line.
An example of a protection system of this type is illustrated in FIG. 3. The equipment to be protected is, for example, a collective equipment (CE) 6 of telephone exchange type having a ground connection 7. Each conductor T, R of the twin-wire telephone line is then associated to a series protection device 2 (PTC). Further, each input terminal 4, 5 of equipment 6 is connected, by a protection device of parallel type 3, to ground 7.
A disadvantage of conventional protection devices such as discussed hereabove is that the current of a lasting disturbance (for example, a current induced by the vicinity of a conductor connected to the mains) may not be sufficient to trigger the protection device of series type but cause a heating of the parallel protection device 3.
Indeed, as shown in previously-described FIG. 2, a conventional parallel protection device only triggers when the voltage thereacross has reached its threshold voltage Vb0. However, before the brick-over point, in an area between a voltage Vbr and voltage Vb0, device 3 starts by an avalanche phase. Accordingly, if the current carrier by the line is in this intermediary range, the parallel protection device will dissipate power and risks being damaged.
Another disadvantage of the conventional protection system is that, on the collective equipment side, the telephone line must be balanced, that is, the equipment input series impedance must be the same on both line conductors. Now, the protection components with a positive temperature coefficient that are generally used, which have an idle series resistance on the order of 10 ohms, have a tolerance of ±20%. An imbalance between the protection devices of the two conductors alters the performances of the collective equipment.
Another disadvantage of the conventional solutions is that they are not integrable.